The present invention relates to an amorphous film solar cell of a p-i-n heterojunction type, which is improved in characteristics through the combined use of at least more than two types of amorphous semiconductor materials.
In recent years, a method of effectively using solar energy has been put into practice. The amorphous solar cells using amorphous silicon film semiconductors are being developed as a lower cost alternative to crystalline solar cells using conventional silicon monocrystal semiconductors (100 to 500 .mu.m in thickness).
The amorphous solar cell is as thin as approxmately 1 .mu.m so that its preparation can conserve energy and materials. Furthermore, the amorphous solar cell can be provided through a relatively simple manufacturing process. Accordingly, the amorphous solar cells are being actively developed as lower cost solar cells.
As this type of amorphous film solar cell, hydrogenated amorphous silicon semiconductor (hereinafter referred to as a a--Si:H) film solar cells, produced by a glow discharge method using monosilane (SiH.sub.4) gas as a main component, and fluorinated amorphous silicon semiconductor (hereinafter referred to as a--Si:F:H) film solar cells containing an amorphous alloy of silicon, fluorine and hydrogen produced by a glow discharge method using silicon tetrafluoride (SiF.sub.4) gases, as main components are being developed.
The amorphous film solar cell of small area using a--Si:H of the former type provides 5.5% photoelectric conversion efficiency in a Schottky construction and 4.5% in a p-i-n construction. The amorphous film solar cell using the a--Si:F:H of the latter type provides 5.6% or more photoelectric conversion efficiency in a Schottky construction.
As for the construction of the amorphous film solar cells, various kinds are known, such as the p-i-n, Schottky and MIS (metal-insulator-semiconductor) type constructions. The p-i-n type construction is more desirable than the others as a lower-cost film solar cell. This is because series-parallel connection of the unit elements is easier to perform on the same base plate. To construct the p-i-n type film solar cell, it is desirable to use amorphous semiconductors of p type, i (intrinsic) type and n type, which are superior in electrooptical properties. The a--Si:F:H film which is being developed at the present time is a much superior material for the i type or n type semiconductor. However, it has been found out through experiments that the a--Si:F:H film, when a p type impurity B (boron) or the like is added thereto, does not become a p type semiconductor which is superior in electrooptical properties.